Wind power generator systems regulate the position of the rotor blades using converter-equipped drives acting as pitch-control drive systems. So that the maximum load carrying capacity of the converters is not exceeded, the actual current at any given time is monitored. The permitted limiting current of the converters is so designed that a dynamic peak current is permitted for a defined length of time and is then reduced to the rated current of the units. It is known that the current permitted under dynamic conditions is equal to twice the rated current of the given unit. Typical drives allow 1.5 to 2 times the rated current of the unit a usual value for the dynamic current.
The converters which are used for D.C. drives are typically four-quadrant converters having B6 thyristor bridges connected in parallel, but oriented in opposite directions to one another. The dynamic current typically is available for 6 seconds in the converter. However, a period of 6 seconds is only achieved if there is no pre-existing load whatever on the converter. A reduction then takes place to the continuous current or rated current which is set.
The value of the dynamic current and its duration are preset at fixed values, with the monitoring of the current being performed by means of a monitoring circuit. With certain pre-existing loads, it may happen that all that the converter will then permit is the rated current. Depending on the load torque, this may result in the required revolution speed not being reached, which may result in the drive stopping. Particularly if there are resistances in the mechanical transmission, it may thus happen that the drive stalls and that it is erroneously switched off by the master fault monitoring system.